1. Field of the Invention
This invention relates to a system for automatically controlling the intermittent pumping of a well. More particularly, the present invention relates to a control system which automatically deactivates the pump for a preset period just prior to an expected fluid pound, reactivates the pump, and automatically readjusts the pumping time after a predetermined number of cycles in a control mode.
2. Description of the Prior Art
Many wells, particularly oil wells, employ a reciprocating pump installed generally adjacent the fluid level of the reservoir being pumped such that the lower end of the pump is submerged. The pump comprises a plunger which is connected to a drive mechanism through a sucker rod extending upwardly through the well and out a wellhead at the upper end of the well. At its upper end, the sucker rod is coupled to a beam-type pumping unit driven by an electric motor or internal combustion engine.
Initially, a pump is operated continuously because of the volume of the oil in the underground reservoir penetrated by the well. After the initial operating period, the reservoir is partially depleted to an extent that the maximum pumping capacity of the pump is greater than the flow of fluid into the bore hole of the well from the reservoir. The pump is then operated intermittently to compensate for the reduced flow into the bore hole.
The intermittent operation of the pump should be controlled to avoid a "pump-off" condition and fluid pound caused by the pump-off condition. Additionally, the controlled cycling of the pump should maximize well production and pump efficiency. When a well is in a pump-off condition or is over-pumped, the fluid level in its annulus falls to a point such that the pump only is partially filled with fluid during the upstroke of its operation. During the downstroke, the movable portion of the pump (pump plunger or travelling valve) will hit the fluid surface causing a fluid pound. The fluid pound causes compression and strain waves in the sucker rod, repeated occurrence of which can cause premature failure of the rod pumping equipment. The reduced flow during the pump-off condition also reduces the efficiency of the pumping mechanism.
Numerous systems have been developed for sensing the pump-off condition and then turning off the pump. The pump-off detection methods involve sensing motor current, annulus fluid level, vibration of the rod string, polished rod load fluctuations and motor or polished rod power fluctuations.
The system which monitors the polished rod load is the preferred means of detecting fluid pound. Such system comprises a strain gauge transducer fixed to the walking beam of the pump unit for detecting changes in the polished rod load through deflections in the walking beam. The sensor transducer transmits a signal proportional to the load on the rod during a predetermined, early portion of the pump-down stroke. During a pump-off condition, the load on the rod is increased when compared to that of normal pump operation. Thus, if the load on the rod is over a predetermined amount, the pumping unit is shut down for a preset down time. A typical example of this type of pump-off control is disclosed in U.S. Pat. No. 3,851,995 to Mills.
Such pump-off control systems are disadvantageous since actuation requires the occurrence of a fluid pound. Thus, such systems cause the pump unit to be subjected to the fluid pound stresses prior to deactivation, rather than anticipating the occurrence of a pump-off conditon or fluid pound and deactivating the pump prior to such occurrence.
U.S. Pat. No. 2,456,456 to Smith discloses a system for setting optimum pumping and pump down times. Such system involves operating the pumping equipment for a time greater than that necessary to deplete the fluid in the well while recording the energy consumption of the pumping equipment. After the pump has been shut down for an arbitrary time period, the pumping and recording steps are repeated. From the records generated, the optimum pumping and shut down times are determined such that the well can be adjusted for automatic pumping operation according to the calculated optimum times.
In another system disclosed in U.S. Pat. No. 4,311,438 to Comstedt, the intermittent operation of a well pump is controlled by a pump-off control which shuts the pump down upon sensing the pump-off condition. The running time of the pump is measured and used to vary the subsequent down time of the pump. A long running time results in a short down time, while a short running time gives a long down time.
Conventional systems for automatically controlling intermittent pumping operations are disadvantageous in that they do not operate the pump at a sufficiently high efficiency level and/or do not adequately avoid the occurrence of fluid pounds. Inefficient pump operation wastes engergy in operating the pump and reduces output of the well. By failing to avoid the occurrence of fluid pound, the pumping mechanisms controlled by the conventional systems tend to suffer a higher failure rate.